A photovoltaic cell is an electrical device formed of semiconducting material able to absorb radiant light energy and convert it directly into electrical energy. Some photovoltaic cells are employed as sensors in cameras and the like to obtain an electrical signal or measure of the ambient light. Other photovoltaic cells are used to generate electrical power. Photovoltaic cells are typically used to power electrical equipment for which it has otherwise proved difficult or inconvenient to provide a source of continual electrical energy.
An individual photovoltaic cell has a distinct spectrum of light to which it is responsive. The particular spectrum of light to which a photovoltaic cell is sensitive is primarily a function of the material forming the cell. Photovoltaic cells that are sensitive to light energy emitted by the sun are commonly referred to as solar cells. Individually, any given photovoltaic cell is capable of generating only a relatively small amount of power, for example, some solar cells generate power in the range of 0.5 to 3.0 watts. Consequently, for most power generation applications, multiple photovoltaic cells are connected together in series into a single unit, an array. When a photovoltaic cell array, such as a solar cell array, is placed outdoors, a shadow or obstruction can temporarily or permanently cover one or more of its photovoltaic cells so they are blocked from receiving light and are unable to generate power.
Electrically, a conventional photovoltaic cell may be considered equivalent to a current source in parallel with a diode that is forward biased relative to the current source. Problems arise in a series-connected photovoltaic cell array when an individual cell ceases to generate electricity because the diode characteristics of the inactive photovoltaic cell cause that cell to appear as a reverse-biased diode to the other cells. The inactive cell blocks current flow and a large voltage develops across the cell. Consequently, the inactive photovoltaic cell, instead of functioning as a current source, functions as a power sink that can consume most of the energy produced by the remaining active photovoltaic cells. The power output of the array falls dramatically. Moreover, the power consumed by the inactive photovoltaic cell is converted into heat, which damages the photovoltaic cells and/or the surrounding components to promote their failure. Also, exposure to large voltages for extended periods of time across an individual photovoltaic cell can cause the semiconducting material forming the cell to break down permanently, resulting in a cell failure.
To prevent these problems, arrays customarily include a forward-biased diode, i.e., a bypass diode, connected in parallel across sets of one or more of the cells that form the array. In the event a cell is rendered inactive, current will flow through the bypass diode eliminating the loss of power and the generation of heat.
While bypass diodes have proved a useful addition to photovoltaic cell arrays, they have their own disadvantages. Having to provide these diodes adds to the overall cost of any array. Furthermore, the addition of the diodes adds to both the weight and size of the cell array.